36 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 125 
(Holt, 1968) supporting the hypothesis that an eversible penis, as 
opposed to the protrusible one, is primitive. If these arguments be 
allowed, it would be expected that pro-pterodrilus may have been 
provided with a penis that was proportionally longer and less in 
diameter than the cone-shaped one of Cambarincola. Such a penis is 
found in P. missourtensis, P. mexicanus, and P. choritonamus. 
The spermatheca had an ectal duct that was heavily muscular and 
entally expanded at its junction with the spermathecal bulb, which in 
turn was provided with a muscular wall or a thick lining of tall 
collumnar epithelial cells. There may have been an ental process, but 
in any case the spermatheca consisted of more diverse elements than 
the simple muscular tube that is the spermathecal duct and the thin- 
walled expanded bulb without an ental process characteristic of the 
advanced members of the genus and of Cambarincola. This opinion is 
based upon conditions in related but more primitive genera (Holt, 
1960a, 1967b, 1968) and upon a consideration of conditions in what 
are otherwise thought to be primitive species of Pterodrillus, i.e., those 
with an undifferentiated prostate. 
A PHYLOGENY OF THE GENUS Preropritus.—Except that it has low 
dorsal ridges on segments I to VII and that the spermatheca varies 
in ways difficult to evaluate, P. missouriensis fits remarkably well the 
above description of the primitive Pterodrilus. But three other species 
form with this one a group of primitive phylogenetic relicts in the 
genus: P. choritonamus, P. mexicanus, and P. cedrus. The major 
problem remaining in the attempt to reconstruct the history of the 
genus is that of convergence. If one bases a proposed phylogeny on 
the evolution of dorsal ridges and projections, a quite satisfactory 
scheme is produced except that there are two distinct lineages of 
which the more advanced members of each have very similar re- 
productive systems. Conversely, a phylogeny based on the evolution 
of the reproductive systems produces a phylogenetic dendrogram that 
is almost a straight line and places closely together such species as 
P. hobbsi and P. alcicornus that otherwise are unlike. The solution 
has been a modified compromise (fig. 11) that assumes a condiserable 
degree of convergence in the evolution of the reproductive systems, 
mostly because the alternative would suggest that at least some 
limeages alternately acquired and lost dorsal ridges and projections, 
an inherently improbable hypothesis. 
Two levels of structural specialization were reached in the evolution 
of Pterodrilus and two minor radiations occurred. Four species (P. 
missouriensis, P. choritonamus, P. mexicanus, and P. cedrus) compose 
a group, derived from the original pro-pterodrilus stock, that is 
characterized by primitive features of the reproductive system and 
dorsal projections on only one segment or none at all. From the 
